In a wireless communications network, devices need to use a frequency resource to transmit information. The frequency resource is also referred to as a spectrum or a frequency band. The frequency band may include an authorized frequency band and an unauthorized frequency band. The unauthorized frequency band is also referred to as an unlicensed frequency band. The authorized frequency band is a dedicated frequency resource of some operators. The unlicensed frequency band is a common frequency resource in the wireless communications network. With the development of communications technologies, an increasing amount of information is transmitted in the wireless communications network. Transmitting information by using the unlicensed frequency band can improve a data throughput in the wireless communications network and better meet user requirements.
Using an Licensed-Assisted Access Using LTE (LAA-LTE) system as an example, an LAA-LTE technology is mainly intending to use a Carrier Aggregation (CA) configuration and structure in an existing Long Term Evolution (LTE) system, to configure, on the basis of configuring a carrier on a licensed frequency band (briefly referred to as a licensed carrier in this specification) of an operator for communication, multiple carriers on an unlicensed frequency band (briefly referred to as unlicensed carriers in this specification), and use the unlicensed carriers for communication with the assistance of the licensed carrier. That is, an LTE device may use a CA manner, to use a licensed carrier as a Primary Component Carrier (PCC) or a primary cell (PCell), and use an unlicensed carrier as an secondary component carrier (SCC) or an secondary cell (SCell). In this way, the LTE device not only can inherit, by using the licensed carrier, conventional advantages of the LTE device in wireless communication, for example, advantages in such aspects as mobility, security, quality of service, and simultaneous scheduling processing for multiple users, but also can implement network capacity offloading by using the unlicensed carrier, to reduce load of the licensed carrier. When using an unlicensed frequency band resource, an LAA system needs to obey specifications formulated by various regions for using an unlicensed frequency band.
Description of an Unlicensed Frequency Band:
Resource sharing on an unlicensed frequency band refers to that only limitations on indexes such as transmit power and out-of-band leakage are set for use of a particular spectrum, to ensure that a basic coexistence requirement is met between multiple devices that use the frequency band, and a radio technology, an operating enterprise, and a service life are not limited, but quality of service on the frequency band is not ensured. An operator may implement network capacity offloading by using an unlicensed frequency band resource, but needs to obey regulations and requirements of different regions and different spectrums for the unlicensed frequency band resource. These requirements are generally formulated to protect a common system such as a radar and ensure that multiple systems do not impose harmful impact to each other as far as possible and fairly coexist, and include a transmit power limitation, an out-of-band leakage index, and indoor and outdoor use limitations, and some regions further have some additional coexistence policies and the like.
Analysis on a Coexistence Specification of an Unlicensed Frequency Band:
For an unlicensed target frequency band that LAA-LTE considers to use, an listen before talk (LBT) coexistence specification needs to be obeyed in some regions and countries, for example, Europe and Japan. The listen before talk LBT is a coexistence policy between systems. When a wireless communications device (for example, for an LTE or LAA-LTE system, the wireless communications device may include a base station and user equipment) occupies the unlicensed frequency band for communication, a detect before use (that is, the LBT) rule needs to be used first. A basic idea of the LBT is: Before sending a signal on a channel, each communications device needs to first detect whether a current channel is idle, that is, whether it can be detected that a nearby node is occupying the channel detected by the communications device (that is, the current channel) to send a signal. This detection process may be referred to as a clear channel assessment (CCA). If it is detected within a period of time that the channel is idle, the communications device can send a signal. If it is detected that the channel has been occupied, the communications device currently cannot send a signal. In the foregoing process, the detecting whether a channel is idle may be implemented through signal detection, energy detection, or the like. Correspondingly, if no particular signal is detected, for example, for a Wireless Fidelity (Wi-Fi) system, the particular signal may be a preamble signal Preamble, it may be considered that the channel is idle. If energy detection is used, if received or detected energy is lower than a given threshold, it may also be considered that the channel is idle. Referring to FIG. 1, FIG. 1 is a schematic diagram of opportunistic data transmission on an unlicensed frequency band in LAA-LTE according to the prior art. In FIG. 1, based on the characteristic of the LBT, data transmission of an LTE device on the unlicensed frequency band is opportunistic, that is, not continuous.
To effectively use an unlicensed frequency band resource to transmit data and improve spectrum utilization efficiency, on an unlicensed frequency band, an LTE system may use a time resource less than one subframe (a partial subframe) and a frequency resource to transmit data, as shown in FIG. 2. FIG. 2 is a schematic diagram of opportunistic data transmission in a partial subframe on an unlicensed frequency band in LAA-LTE according to the prior art. In FIG. 2, data transmission in a partial subframe (that is, less than one subframe) in the LAA-LTE on the unlicensed frequency band is opportunistic. A time length of the partial subframe is generally less than 1 ms, a time length of a complete subframe is generally 1 ms. For example, when time and frequency resources included in the partial subframe are all used in downlink data transmission, a time length of the partial subframe that is used in the downlink data transmission is less than 1 ms.
Because a length of a partial subframe is less than 1 ms, impact is caused to sending of a reference signal of an LTE system. The reference signal herein includes a discovery reference signal (DRS), a cell-specific reference signal (CRS), a channel state information reference signal (CSI-RS), and the like. The reference signal may be used for an radio resource management (RRM) measurement, and may also be used for a channel state information (CSI) measurement. Using a DRS as an example, for opportunistic sending of data on an unlicensed frequency band, to resolve an radio resource management (RRM) measurement problem of user equipment (UE), the discovery reference signal (DRS) is used on the unlicensed frequency band to support an RRM measurement of the unlicensed frequency band. The RRM measurement herein includes a measurement for a serving cell and/or a neighboring cell by the UE, for example, an RSRP (Reference Signal Received Power, reference signal received power) measurement, an reference signal received quality (RSRQ) measurement, or an received signal strength indicator (RSSI) measurement. Considering an inter-frequency measurement problem, the DRS is generally sent in a configured discovery signals measurement timing configuration (DMTC), and duration of the DMTC is 6 milliseconds. The DRS includes a primary synchronization signal (PSS), an secondary synchronization signal (SSS), a CRS, and a configurable CSI-RS. A time range including DRS sending is generally referred to as a DRS Occasion or DRS occasion duration, and may be represented by an integer quantity of orthogonal frequency division multiplexing (OFDM) symbols, or may be represented by an integer quantity of subframes. For example, assuming that the DRS occasion duration is one subframe, a representation form of time and frequency resources for sending a DRS is shown in FIG. 3. FIG. 3 is a schematic diagram of a DRS according to the prior art. As seen from reference signals included in the DRS, if the DRS does not include a configurable CSI-RS, the DRS may be less than 1 ms in time, that is, may include only 12 OFDM symbols. Herein, that the DRS includes 12 OFDM symbols in time is described with respect to a quantity of OFDM symbols occupied by a start position to an end position of the DRS. In FIG. 3, within one subframe (1 ms), the start position of the DRS is a first symbol (carrying a CRS), and the end position of the DRS is a twelfth symbol (carrying a CRS). Therefore, the DRS includes 12 OFDM symbols in time. It should be noted that FIG. 3 is considered for a case in which a downlink data transmission configuration is a normal cyclic prefix, and FIG. 3 shows only REs (Resource Element, resource element) occupied by a DRS in time and frequency resources consisting of 12 subcarriers and 14 OFDM symbols (corresponding to a length of one subframe in the normal cyclic prefix configuration), where the DRS includes a PSS, an SSS, and a CRS.
Generally, when UE executes an RRM measurement, especially an RRM measurement of a neighboring cell (which is not a serving cell of the UE), the UE first determines, on a target frequency band based on whether a PSS and an SSS are detected, whether a target cell exists on the target frequency band. Herein, a carrier on which the target cell is located is the target frequency band. Then the UE may determine whether there is a DRS in a DMTC, and if the UE detects a PSS and an SSS in the DMTC, the UE determines that there is a DRS in the DMTC (because a DRS includes a PSS and an SSS). Then the UE uses a reference signal included in a DRS occasion (a time range for DRS sending), such as a CRS, or a CRS and a CSI-RS, to execute the RRM measurement. This causes a problem, that is, in a case in which an LAA-LTE system performs opportunistic data transmission and supports data transmission in a partial subframe, due to transmission in the partial subframe, complete DRS data sending and receiving cannot be ensured, causing the UE to misinterpret the DRS, which directly causes an incorrect RRM measurement when the UE executes an RRM measurement of a neighboring cell.
Similarly, the partial subframe also affects sending of a CSI-RS. Currently, transmission in a partial subframe also occurs in an LTE system. That is, in a time division duplexing (TDD) system, a data transmission length of a downlink pilot timeslot (DwPTS) included in a special subframe is less than 1 ms, and for user equipment in an LTE system release 12, CSI-RS transmission is not supported in the DwPTS. Generally, a CSI-RS resource is periodically configured. For opportunistic transmission on an unlicensed frequency band, if the partial subframe does not support CSI-RS data transmission, either, sending of a periodic CSI-RS may be missed, and a measurement by the user equipment on channel state information of the unlicensed frequency band is affected.
In conclusion, for an LTE system working on an unlicensed frequency band, in the case of opportunistic data transmission, to improve spectrum utilization efficiency, transmission in a partial subframe may be used. When the transmission in a partial subframe is used, how to set a data transmission feature of the partial subframe to ensure that user equipment correctly interprets a reference signal in the partial subframe and ensure an accurate RRM and/or CSI measurement is an important problem to be resolved.